Bump wikimedia/parsoid to 0.21.0-a15

Bug: T385817
Depends-On: I7abbf3a834f27135934f037077399a5da052b596
Change-Id: I38ba0453b15f6b545261779dd4d271bf55412ec9

Author: subbuss

composer.json

@@ -69,7 +69,7 @@
 		"wikimedia/minify": "2.8.0",
 		"wikimedia/normalized-exception": "2.1.1",
 		"wikimedia/object-factory": "5.0.1",
-		"wikimedia/parsoid": "0.21.0-a14",
+		"wikimedia/parsoid": "0.21.0-a15",
 		"wikimedia/php-session-serializer": "3.0.0",
 		"wikimedia/purtle": "2.0.0",
 		"wikimedia/relpath": "4.0.1",